This invention relates to reduced pressure treatment of molten metal such as molten aluminum to reduce the amount of gaseous and volatile material contained therein.
Molten metal contains dissolved and suspended impurities. Molten aluminum, for example, contains dissolved impurities such as alkali and alkali earth metals or elements such as hydrogen.
As used herein, the term molten metal, e.g., molten aluminum, is intended to mean the molten metal and its alloys.
Because of its atomic size, hydrogen has significant solubility in molten metal and in particular in molten aluminum. In high purity molten aluminum, hydrogen solubility at slightly above its melting temperature is about 0.9 cc/100 gm aluminum at standard temperature and pressure. However, upon solidification of the aluminum, hydrogen solubility is decreased by a factor of about 10. The solubility difference between molten and solid aluminum results in hydrogen gas forming, a portion of which is retained or trapped in the solid aluminum. The trapped hydrogen gas results in porosity in the aluminum ingot that is very detrimental to properties of products fabricated therefrom. Thus, it is highly desirable to remove dissolved hydrogen or other volatile elements such as sodium, potassium or calcium, etc., from molten aluminum to a very low level in order to reduce porosity in the resulting ingot and its attendant problems.
Hydrogen has been removed from molten aluminum by treatment with various gases where such gases are injected into the melt in small bubble form. Hydrogen in the melt migrates to the gas bubble and is carried out of the melt with the gas bubble. However, to reduce the hydrogen to low levels requires the use of large quantities of gas that can result in the generation of skim.
Russian disclosure 431,962 discloses that large dimension castings of improved quality can be made in metal molds by vacuum degassing each separate section of the mold after it is filled with melt. The sections of the mold are air-tight and connected with the working volume of the mold by a gas-permeable component. Air and gas are drawn through the gas-permeable component and vacuum is maintained at a level to remove gaseous substances but excludes drawing liquid metal into the pores of the gas permeable component.
European Application 91105145.6 discloses a vacuum degassing method and an apparatus therefor wherein a vacuum pump is used to draw gas from a melt or gas produced by reaction at the interface between the melt and a porous member, through the porous member to the side of the porous member not in contact with the melt and kept under vacuum. According to this disclosure, materials used for the porous member are metal oxides or other ceramic compounds (non-oxides) and mixtures thereof such as Al.sub.2 O.sub.3, MgO, CaO, SiO.sub.2, FeO, Cr.sub.2 O.sub.3, BN and Si.sub.3 N.sub.4. However, one problem with such ceramic materials is that they are subject to thermal shock which leads to degradation. Materials such as silica are reactive with molten metal such as molten aluminum and this leads to severe contamination. Further, many of these materials are costly and difficult to fabricate. Machining of such material is difficult and grain fracture can occur which leads to difficulty in controlling pore size. This has the disadvantage that it greatly interferes with the efficiency and effectiveness of the porous member.
The present invention overcomes these problems by providing a porous member that is highly resistant to thermal shock, easily fabricated and highly resistant to attack by molten metal such as molten aluminum. The present invention provides a method and apparatus for removing dissolved gases such as hydrogen and volatile materials which can include sodium, calcium, potassium, zinc and magnesium from molten metal such as molten aluminum.